1. Field of the Invention
The present invention relates to an optical fiber and an optical fiber device.
2. Description of the Related Art
A highly nonlinear optical fiber is an optical fiber having a larger nonlinear coefficient as compared with a standard optical transmission fiber in an optical communication system (see, e.g., M. Onishi, ECOC'99, pp. II-216-II-219). Such a highly nonlinear optical fiber is used, as a medium for generating a nonlinear optical phenomenon, in optical devices such as an optical switch, an optical amplifier, and an optical generator. It is known that performance of the optical devices is restricted by generation of stimulated Brillouin scattering (SBS).
The term “stimulated Brillouin scattering” means a phenomenon that when light having power over a certain threshold is launched into an optical fiber, a part of the light is reflected and, therefore, the light cannot propagate through the fiber at high intensity. In order to enhance the efficiency of generating nonlinear optical phenomena, an effective area of highly nonlinear optical fiber is designed to be small to increase the power density of transmitting light. This design makes the stimulated Brillouin scattering to be generated more easily as compared with the standard optical transmission fiber. As a result, the intensity of the transmitted light is restricted and the efficiency of the optical device is restricted.
In view of the above-described problem, J. Hansryd, et al., J. Lightwave Techn. Vol. 19, pp. 1691-1697(2001) (Document 1) and U.S. Pat. No. 5,170,457 propose techniques for suppressing the generation of the stimulated Brillouin scattering in the highly nonlinear optical fiber. The technique described in Document 1 is to increase a threshold of the stimulated Brillouin scattering in a manner in which a temperature gradient applied to the optical fiber is utilized. The technique described in U.S. Pat. No. 5,170,457 is to increase a threshold of the stimulated Brillouin scattering in a manner in which the optical fiber is designed such that acoustic waves are not guided through a core.
However, the technique described in Document 1 is not satisfactory from the practical point of view because large energy had to be consumed to hold the desired temperature and a large-scale arrangement is required. U.S. Pat. No. 5,170,457 suggests that concentration of Al2O3 included as a dopant in the optical fiber is preferably held within a range not exceeding 8 wt %. In other words, U.S. Pat. No. 5,170,457 does not take into consideration the concentration of Al2O3, i.e., 12 wt %, which is required to obtain large nonlinearity.